The Government's Brexit secretary David Davis has resigned in a major blow to Prime Minister Theresa May and her exit strategy from the European Union.

The Prime Minister is hoping to win over Tory Brexiteers after a backlash against her plans for leaving the European Union.

But Mr Davis, who signed up to the plan agreed by the Cabinet at Chequers on Friday, has now quit. Two senior sources confirmed Mr Davis' departure from the Cabinet.

His junior ministerial colleague Steve Baker has also quit his role at the Department for Exiting the European Union (Dexeu) along with Suella Braverman, another Brexit minister.

The resignations are likely to embolden Brexit supporting backbenchers with concerns about Mrs May's leadership.

Great Grimsby Labour MP Melanie Onn tweeted that Mrs May was now vulnerable.

She said: "Who’ll be handed this poisoned chalice & who’s brazenly ambitious/end of career to take it on? If they still have same Leader of course. Which they May not."

The move comes on the eve of a major test for the Prime Minister as she faces first the House of Commons and then a potentially stormy meeting of Tory MPs and peers on Monday.

Mrs May is expected to tell MPs that the strategy agreed on at Chequers is the "right Brexit" for Britain.

Mr Davis had come close to resigning before, but Mrs May must have hoped the danger of Cabinet resignations had passed after Friday's deal at her official country retreat.

Brexiteer Cabinet minister Michael Gove admitted the plan was not everything he had hoped for, but he was a "realist" and the Prime Minister's lack of a Commons majority meant the "parliamentary arithmetic" was a factor in deciding what could be adopted.

David Davis's resignation letter in full

Dear Prime Minister

"As you know there have been a significant number of occasions in the last year or so on which I have disagreed with the Number 10 policy line, ranging from accepting the Commission's sequencing of negotiations through to the language on Northern Ireland in the December Joint Report.

"At each stage I have accepted collective responsibility because it is part of my task to find workable compromises, and because I considered it was still possible to deliver on the mandate of the referendum, and on our manifesto commitment to leave the Customs Union and the Single Market.

"I am afraid that I think the current trend of policy and tactics is making that look less and less likely.

"Whether it is the progressive dilution of what I thought was a firm Chequers agreement in February on right to diverge, or the unnecessary delays of the start of the White Paper, or the presentation of a backstop proposal that omitted the strict conditions that I requested and believed that we had

agreed, the general direction of policy will leave us in at best a weak negotiating position, and possibly an inescapable one.

"The Cabinet decision on Friday crystallised this problem.

"In my view the inevitable consequence of the proposed policies will be to make the supposed control by Parliament illusory rather than real.

"As I said at Cabinet, the "common rule book" policy hands control of large swathes of our economy to the EU and is certainly not returning control of our laws in any real sense.

"I am also unpersuaded that our negotiating approach will not just lead to further demands for concessions.

"Of course this is a complex area of judgement and it is possible that you are right and I am wrong.

"However, even in that event it seems to me that the national interest requires a Secretary of State in my Department that is an enthusiastic believer in your approach, and not merely a reluctant conscript.

"While I have been grateful to you for the opportunity to serve, it is with great regret that I tender my resignation from the Cabinet with immediate effect.

Foreign Secretary Boris Johnson backed the proposals at Chequers, despite claiming that defending the plans was like "polishing a turd" during the meeting.

But resistance to the plan from hardline Eurosceptics has been growing over the weekend.

Arch-Brexiteer Jacob Rees-Mogg hit out at the "defeatism" in the Government's plans, warning that he would vote against them - and suggested other Eurosceptics may do the same.

In the Commons, Mrs May will acknowledge that there have been "robust views" around the Cabinet table and a "spirited national debate" since the 2016 referendum decision to leave the EU.

She will say: "Over that time, I have listened to every possible idea and every possible version of Brexit. This is the right Brexit."

She will tell MPs it was "the Brexit that is in our national interest" and "will deliver on the democratic decision of the British people".

The exit of David Davis is a bitter blow to Prime Minister Theresa May (Image: Carl Court/Getty Images)

The Prime Minister will insist the plan, which would see the UK share a "common rulebook" for goods as part of a proposal to create a UK-EU free trade area, still meets her Brexit red lines.

Mr Davis' decision could now act as a spur to other Brexiteers to take action against Mrs May.

Letters calling for a leadership contest have reportedly been submitted to the backbench 1922 Committee.

Tory MP Andrew Bridgen told the Press Association: "I can't support the offer which emerged at Chequers - I think it's a breach of the red lines, in fact the offer is so poor that I couldn't support it even if the EU were paying us for it.

"Obviously if the Government and the Prime Minister continue to support that very poor offer then I won't have any confidence in the Government or the Prime Minister."

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Veteran Tory Eurosceptic Sir Bill Cash told Sky News "there is a lot of unhappiness" with MPs asking: "Is this going to be a proper Brexit?"

He said he had not written a letter calling for a leadership contest, but pointed out "if people were to decide to put in those letters you only need 48".